The present invention relates to an encapsulated longitudinal measuring device which is particularly adapted for measuring large displacements. This device comprises a flexible scale which is mounted in a longitudinally displaceable manner in a hollow tubular member. This tubular member also encloses a sensing unit which moves along the scale and measures its position on the scale. Generally, an evaluation and display unit is also included to process the position information generated by the sensing unit This measuring installation may be used to measure a relative movement, for example, the relative movement between two machine components such as a machine bed and a machine carriage. The scale and the scanning unit are generally high precision components which are protected from environmental influences by the tubular member which surrounds them.
In high precision measuring systems the selection of the component materials used in a particular device is highly important, especially since materials with different expansion coefficients often must be combined. Ideally, it would often be desirable to use materials having identical or similar expansion coefficients, however, cost considerations often make this impractical. Most frequently a measuring system will combine machine components made from grey castings, a hollow tubular member of aluminum and a scale made of steel or glass.
Certain measuring devices of the prior art connect the measuring scale to the basic body of the device in a longitudinally displaceable manner, for example, by way of a highly elastic intermediate layer. The scale is thereby isolated in some measure from changes in length due to bending of the tubular member (German Patent No. 1,176,382). Moreover, a measuring device is shown in U.S. Pat. No. 3,816,002, in which a housing is fastened to the carriage of a machine tool. The scale is fixed within the housing at one end, while the other end of the scale is positioned in a kind of tensioning apparatus within the housing. This tensioning apparatus operates to compensate by way of a spring for the temperature-dependent longitudinal changes of the housing. As a result of this arrangement the scale remains relatively uninfluenced by temperature-dependent longitudinal changes of the housing. One of the disadvantages of the last-named arrangement, however, is that the scale is tensioned freely between suspension points at both of its ends in the interior of the housing. Vibrations associated with the operation of machine tools can act on the scale and cause it to vibrate. In some cases this vibration may cause errors and even damage to the scanning unit. Therefore, this method is in general applicable only conditionally and then for only relatively small measuring lengths.